Posterior capsular opacification (PCO) and optic decentration are two of the most common complications of modern cataract surgery and intraocular lens (IOL) implantation. The most common cause of PCO is believed to relate to the proliferation and migration of retained lens epithelial cells and their derivatives into the visual axis. The presence of optic components such as positioning holes or optic edges within the pupillary aperture, arising from lens decentration, can cause occupationally debilitating visual aberrations.
Animal studies have shown that the following features having a significant statistical impact on reducing PCO for capsular fixated IOLs: (1) one-piece, all polymethylmethacrylate (PMMA) IOL, (2) biconvex or posterior convex optic design, and (3) angulated loops. S. 0. Hansen, Posterior Capsular Opacification And Intraocular Lens Decentration, Journal Of Cataract And Refractive Surgery Vol. 14, No. 6, pp. 605-623 (November 1988). Also, the most consistent centration is apparently obtained using one-piece, PMMA IOLs. Optic design (shape and configuration of the optic disc) apparently does not affect lens centration but is significantly related to PCO.
It has been shown that a biconvex optic design and 10.degree. angulated loops, for a one-piece, PMMA posterior chamber IOL, produce the lowest PCO among various optic designs. Hansen, supra. IOLs having a laser ridge optic, meniscus optic (posterior-concave), plano-convex optic (posterior convex or reverse optic) and biconvex optic apparently reduce PCO by inhibiting or minimizing migration of proliferating lens epithelial cells into the visual axis by creating a barrier effect. The barrier effect is best achieved when there is a gentle, taut radial stretch on the capsular sac. PMMA loops apparently have better retention of structural memory and therefore increase the radial stretch of the equatorial capsule that places the capsule under tension and increases the contact of the optic disc with the posterior capsule. This may explain why one-piece, PMMA IOLs have been found to lower PCO irrespective of optic design.
In recent animal studies, a prototype compressible disc IOL (CDIOL) (1) made of onepiece, PMMA, (2) including a biconvex optic, and (3) including an outer fixation ring angled at 10.degree. from the optic, was found to effectively reduce PCO. Hansen, supra. The CDIOL is also described in "Animal Studies Back Search For `Optimal IOL`", Ocular Surgery News, Vol. 6, No. 23 (Dec. 1, 1988). The prototype CDIOL is fabricated from a disc-shaped sheet of solid PMMA. Using a computerized lathe-cutting technique, the lens is fashioned to a biconvex optic disc attached by two connecting bars to an outer fixation ring. The ring has no free end. The lens has a single positioning hole close to the ring to facilitate placement of the superior or twelve o' clock portion of the lens.
Since the outer fixation ring of the CDIOL is coupled to the optic disc by two connecting bars angled at 10.degree. with respect to the disc, the outer fixation ring is situated in a plane anterior to the optic disc. It is believed that the CDIOL effectively reduces PCO by creating a radial stretch of the posterior capsule which is induced by (1) the outer fixation ring which extends 360.degree. around the disc, (2) the angulation of the connecting bars, and (3) the biconvex optic disc. These structural features are believed to maintain the posterior capsule taut while pushing the optic firmly against the posterior capsule, thus providing a mechanical barrier against cell proliferation. Hansen, supra. It is recognized in Hansen, supra, that an IOL with broad circular loop design and angulation, such as the CDIOL or a long C-loop lens in a one-piece design, may prove to further inhibit PCO and decentration upon further study.